The present invention relates to an ink for producing membrane electrode assemblies for fuel cells, in particular for polymer electrolyte membrane fuel cells (PEM fuel cells) and direct methanol fuel cells (DMFC). More particularly, the present invention relates to a water-based catalyst ink for producing membranes coated with catalyst, electrodes and membrane electrode assemblies (MEAs).
Fuel cells convert a fuel and an oxidising agent which are spatially separated from each other at two electrodes into electricity, heat and water. Hydrogen or a hydrogen-rich gas may be used as the fuel and oxygen or air as the oxidizing agent. The process of energy conversion in the fuel cell is characterised by particularly high efficiency. For this reason, fuel cells in combination with electric motors are becoming more and more important as an alternative to traditional internal combustion engines. The PEM fuel cell is suitable for use as an energy converter in motor vehicles because of its compact structure, its power density and its high efficiency.
The PEM fuel cell consists of a stacked arrangement (“stack”) of membrane electrode assemblies (MEAs), between which are arranged bipolar plates for supplying gas and removing electricity. A membrane electrode assembly consists of a solid polymer electrolyte membrane, both sides of which are provided with reaction layers containing catalyst. One of the reaction layers is designed as an anode for the oxidation of hydrogen and the second reaction layer is designed as a cathode for the reduction of oxygen. To these reaction layers are applied so-called gas distributor layers made of carbon paper or carbon fleece which facilitate good access by the reaction gases to the electrodes and effective removal of the cell current. The anode and cathode contain so-called electrocatalysts which catalytically support the particular reaction (oxidation of hydrogen at the anode or reduction of oxygen at the cathode). Metals from the platinum group in the periodic system of elements are preferably used as the catalytically active components. In the majority of cases, so-called supported catalysts, in which the catalytically active platinum group metal has been applied in highly dispersed form to the surface of a conductive support material, are used.
The polymer electrolyte membrane consists of proton-conducting polymer materials. These materials are also called ionomers for short in the following description. A tetrafluorethylene/fluorovinylether copolymer with acid functions, in particular sulfonic acid groups, is preferably used. Such materials are sold, for example, under the tradenames Nafion® (E.I. DuPont) or Flemion® (Asahi Glass Co.). However, other, and in particular, fluorine-free, ionomer materials such as sulfonated polyetherketones or arylketones or polybenzimidazoles may also be used. In addition, ceramic membranes and other high-temperature materials may also be used.
The performance data for a fuel cell depend critically on the quality of the catalyst layers applied to the polymer electrolyte membrane. These layers are mostly highly porous and usually consist of an ionomer and a finely divided electrocatalyst dispersed therein. Together with the polymer electrolyte membrane, so-called three-phase intersides are formed in these layers, wherein the ionomer is in direct contact with the electrocatalyst and the gases (hydrogen at the anode, air at the cathode) are introduced to the catalyst particles via the pore system.
To prepare the catalyst layers, ionomer, electrocatalyst, solvent and optionally other additives are carefully blended together to form an ink or a paste. To produce the catalyst layer, the ink is applied by brushing, rolling, spraying, doctor blading or printing either to the gas distributor structure (e.g. carbon fleece or carbon paper) or directly to the polymer membrane, dried and optionally aftertreated. In the case of coating the ionomer membrane with a catalyst layer, the non-catalyzed gas distributor structures are then mounted on the membrane on the anode and cathode sides and a membrane electrode assembly (MEA) is then obtained. If the gas distributor is coated with a catalyst layer, these catalyzed gas distributor structures are placed on the two sides of the ionomer membrane and then compression moulded with this, wherein a MEA is also obtained.
Various ink formulations are disclosed in the patent literature. Thus, in DE 196 11 510 A1, an ink is used to produce membrane electrode assemblies for fuel cells which contains, with respect to the total weight of ink, 3.1 wt. % of a Pt/C catalyst (30 wt. % platinum on carbon black), 30.9 wt. % of a 5% strength ionomer solution in a mixture of 90 parts isopropanol and 10 parts water, 37.2 wt. % glycerine, 24.8 wt. % water, 2.5 wt. % tetrabutylammonium hydroxide and 1.5 wt. % of a pore-former. The water content of the ink is 27.7 wt. % in total. As a result of the high concentration of isopropanol in this ink, appropriate measures have to be taken during production to prevent unwanted ignition of the catalyst. In addition, it has been shown that the ink can be processed only over a very short time by means of a screen printing process due to the low boiling point of isopropanol; the so-called “screen life” during which screen printing is possible is unsatisfactory. Furthermore, the glycerine present in the ink means that the membrane electrode assembly (MEA) requires a very long activation and conditioning period before acceptable electrical performance is obtained.
Furthermore, catalyst inks are know which use alcohols with a boiling point higher than 100° C. (U.S. Pat. No. 5,871,552) or alkylene carbonates such as, for example, propylene carbonate (U.S. Pat. No. 5,869,416) as solvent. Furthermore, DE 198 12 592 A1 describes an ink of two organic solvents A and B which are not miscible with each other. Monohydric or polyhydric alcohols, glycols, glycol either alcohols, glycol ethers and mixtures thereof are used as solvent A. Solvent B is a non-polar hydrocarbon or weakly polar solvent. A typical ink of this type (see example 1 in DE 198 12 592 A1) contains 13.4 wt. % of a Pt/C electrocatalyst, 67 wt. % of a 6.7% strength solution of an ionomer (Nafion) in propylene glycol (solvent A), 17.9 wt % methyl dodecanoate (solvent B) and 1.7 wt. % of sodium hydroxide solution (10% strength). These catalyst inks contain predominately organic solvents and only small amounts of water in the form of the sodium hydroxide solution. Due to the high proportion of solvent, they tend to ignite. The considerable emissions of organic compounds (solvents are “volatile organic compounds”=VOCs) is a problem with regard to occupational health and safety and the protection of the environment, in particular when mass producing components for fuel cells.
EP 0 026 979 A2 describes an ink based on water but which does not contain an ionomer, rather hydrophobized Teflon. This ink is therefore unsuitable for the electrodes and MEAs which are used in PEM fuel cells.
EP 0 731 520 A1 describes an ink which contains a catalyst, ionomer and solvent, wherein water is used as solvent. This ink does not contain any further organic components, apart from the ionomer. When the applicants checked this ink, it was shown that it led to electrode layers which adhered too poorly to the polymer membrane. As a result, the electrical performance of MEAs produced with this ink is inadequate. Likewise, when screen printing with this ink, it was shown that it thickened very rapidly and thus had inadequate screen lifes for screen printing.
Thus, it was an object of the present invention to provide a water-based catalyst ink which contains no toxic and/or readily inflammable solvents and which in addition overcomes the disadvantages of the water-based ink previously described in EP 0 731 520 A1 (poor adhesion, poor electrical performance, short screen life).
Another object of the invention is to attain high production safety in the area of occupational health and safety and protection of the environment and to have an ink particularly suitable for screen printing.